Boron has long been used as a dopant for semiconductor devices. However, boron atoms tend to diffuse during the annealing and oxidation steps of a fabrication sequence. As a result, it is difficult to manufacture devices, such as deep sub-micron NMOS and CMOS device, in which the boron distribution must have a steep profile.
It has long been known that the presence of germanium as a co-dopant will retard the diffusion of boron. For example, U.S. Pat. No. 4,728,619, issued to J. R. Pfiester et al. on Mar. 1, 1988, describes a method for making a CMOS integrated circuit having boron-doped channel-stop regions. Germanium is implanted into these regions to retard the diffusion of boron. The germanium is implanted at a concentration of less than 1 at. %. After the germanium is implanted, a field oxide more than 6000 .ANG. thick is grown.
We believe that the method of Pfiester for providing germanium-doped regions will only be of limited value for making deep sub-micron devices. In order to make boron profiles steep enough for, e.g., vertically engineered devices having buried boron-doped layers, it will be necessary to include more than 1 at. % germanium in the boron-doped regions.
We believe that in the method of Pfiester, there is, in fact, some concentration of the implanted germanium during the subsequent field oxide growth. That is, the advancing oxidation front ejects germanium atoms into the underlying silicon. However, the improvements in device performance reported by Pfiester were measured in devices having more than 6000 .ANG. of field oxide, as noted above. In the manufacture of deep sub-micron devices, by contrast, it would generally be unacceptable to grow more than about 500 .ANG. of oxide at any time after the boron and germanium dopants have been incorporated in the water. That is because, according to at least some generally accepted manufacturing methods, the gross structure of the devices will already have been defined by, e.g., patterning an initial field oxide layer. Subsequent growth of a further oxide layer having a thickness even as small as 500 .ANG. could obliterate this gross structure.
What practitioners in the art have hitherto failed to provide is a method for making MOS devices having germanium-containing regions that will control boron diffusion to such an extent that sharply defined structures such as pulse-shaped or retrograde boron-doped regions are readily incorporated.